The present invention relates to rubber modified polymers obtained from vinyl aromatic monomers.
Rubber modified polymers, such as high impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS), are typically produced by mass polymerizing styrene or styrene/acrylonitrile in the presence of dissolved rubber. ABS is more typically produced using an emulsion polymerization process which produces small rubber particles and high gloss products, but with increased conversion costs.
In the preparation of rubber modified polymers, the rubber particle size and morphology play an important role in controlling the physical properties of the final product. The final rubber particle size can be determined by a number of different parameters including shear, viscosity, and interfacial tension. Increased shear after phase inversion can be used to reduce particle size, however this adds expense and complexity to the process. The final rubber particle size can also be influenced by the viscosity ratio of the disperse phase/continuous phase, and the viscosity of the continuous phase polymer. Sizing readily occurs when the viscosity ratio is between 0.2 and 1; and with higher viscosity of the continuous phase, the greater the ease of particle breakup. The rubber phase viscosity is determined by the rubber level and by the solution viscosity of the rubber. Additionally, grafting and crosslinking of the rubber will increase rubber viscosity. Interfacial surface tension will also influence rubber particle size and morphology, wherein a reduction of the interfacial tension can be achieved by utilizing the compatible block rubbers or by grafting to make compatible rubbers in-situ. Compatible block rubbers are characterized by having a block miscible with the continuous phase and a block miscible with the discontinuous phase. A reduction of the interfacial tension will facilitate the sizing process thereby increasing the flexibility. In HIPS compositions, compatible rubbers include styrene-butadiene block rubbers. In ABS compositions, styrene-butadiene block rubbers are not compatible since polystyrene is not miscible with the SAN continuous phase. SAN-butadiene block rubbers are compatible with ABS, but are not commercially available. Therefore in ABS polymer compositions, compatible block copolymers have to be produced in situ via grafting. The use of functionalized rubbers has been investigated in order to make such compatible block rubbers in-situ in both HIPS and ABS processes due to the economic advantage.
U.S. Pat. No. 5,721,320 by Priddy et al. discloses a free radical polymerization in the presence of a functionalized diene rubber having a stable free radical group such that a vinylaromatic-diene block or copolymer-diene rubber is formed. However, Priddy refers to the production of transparent HIPS and ABS, wherein the rubber particle size is very small (0.1 micron), which is insufficient for many high impact applications.
U.S. Pat. No. 6,262,179 by Atochem discloses a process for producing vinyl aromatic polymers in the presence of a stable free radical. However, the resultant product has a very wide rubber particle size distribution, with a large average rubber particle size, which can is negatively affect physical properties, such as gloss.
U.S. Pat. No. 6,255,402 by Atochem discloses a process of polymerizing at least one vinyl aromatic monomer in the presence of a rubber comprising a group which generates a stable free radical. However, this process utilizes a wide variety of rubbers, including those having high solution viscosity, which can negatively affect physical properties, for example, gloss, of the polymer.
U.S. Pat. No. 6,255,448 by Atochem discloses a process for the polymerization of at least one monomer in the presence of a stable free radical having substitution in the beta position. However, these beta substituted stable free radicals can have increased cost and may not be used in anionic coupling due to the reactivity of the substituent.
WO 99/62975 by Atochem discloses a process using a stable free radical and an initiator. This process also utilizes high viscosity rubbers which can negatively affect gloss and other physical properties.
WO 01/74908 by BASF discloses a method of polymerization in the presence of a stable free radical and a thiol compound. U.S. Pat. No. 4,581,429 discloses the use of alkoxy amines (>N—O—X) in controlled radical polymerization, wherein the alkoxy amine forms a free radical (X.) which is suitable as a free radical initiator and a stable free radical (>N—O.). However, this method does not include the production of rubber modified polymers.
Therefore, there remains a need for an efficient and cost effective mass process for achieving the rubber particle size, distribution and morphology desired, utilizing in-situ produced block rubbers which offer enhanced physical properties and efficient processing.